The history of science is a decision tree.
Vertices appear where we must take one or another branching.
Sometimes, we take the wrong road for the right reasons.

A good example is the geocentric vs. heliocentric cosmology.
The ancient Greeks knew that in many ways it made more sense for the
earth to revolve around the sun than vice-versa. Yet they were very
clever. Ptolemy and others tested for the signature of the earth's orbit in
the seasonal wobbling in the positions of stars, or parallax.
If the earth is moving around the sun, nearby stars should appear
to move on the sky as the earth moves from one side of the sun to the
other. Try blinking back and forth between your left and right eyes
to see this effect, noting how nearby objects appear to move relative
to distant ones.

Problem is, Ptolemy did not find the parallax. Quite reasonably,
he inferred that the earth stayed put. We know now that this was the
wrong branch to choose, but it persisted as the standard world view
for many centuries. It turns out that even the nearest stars are so
distant that their angular parallax is tiny (the angle of parallax is
inversely proportional to distance). Precision sufficient for measuring
the parallax was not achieved until the 19th century, by which
time astronomers were already convinced it must happen.

Ptolemy was probably aware of this possibility, though it must have
seemed quite unreasonable to conjecture at that time that the stars could
be so very remote. The fact was that parallax was not observed. Either
the earth did not move, or the stars were ridiculously distant. Which
sounds more reasonable to you?

So, science took the wrong branch. Once this happened, sociology kicked
in. Generation after generation of intelligent scholars confirmed the
lack of parallax until the opposing branch seemed so unlikely that it
became heretical to even discuss. It is very hard to reverse back up the
decision tree and re-assess what seems to be such a firm conclusion. It took
the Copernican revolution to return to that ancient decision branch
and try the other one.

Cosmology today faces a similar need to take a few steps back on the
decision tree. The problem now is the issue of the mass discrepancy,
typically attributed to dark matter. When it first became apparent that
things didn't add up when one applied the usual Law of Gravity to the
observed dynamics of galaxies, there was a choice. Either lots of
matter is present which happens to be dark, or the Law of Gravity has
to be amended. Which sounds more reasonable to you?

Having traveled down the road dictated by the Dark Matter decision branch,
cosmologists find themselves trapped in a web of circular logic entirely
analogous to the famous Ptolemaic epicycles. Not many of them realize
it yet, much less admit that this is what is going on. But if you take
a few steps back up the decision branch, you find a few attempts to alter
the equations of gravity. Most of these failed almost immediately,
encouraging cosmologists down the dark matter path just as Ptolemy wisely
chose a geocentric cosmology. However, one of these theories is not only
consistent with the data, it actually predicts many important new results.
This theory is known as MOND (MOdified Newtonian Dynamics). It was introduced
in 1983 by Moti Milgrom of the
Weizmann Institute in Israel.

MOND accurately describes the effective force law in galaxies based only
on the observed stars and gas. What this means is unclear, but it clearly
means something! It is conceivable that dark and luminous matter somehow
interact to mimic the behavior stipulated by MOND. This is not expected,
and requires a lot of epicyclic thinking to arrange. The more straightforward
interpretation is that MOND is correct, and we took the wrong branch of
the decision tree back in the '70s.

MOND has dire implications for much modern cosmological thought which has
developed symbiotically with dark matter. As yet, no one has succeeded
in writing down a theory which encompasses both MOND and General Relativity.
This leaves open many questions in cosmology that were thought to be solved,
such as the expansion history of the universe. There is nothing a scientist
hates to do more than unlearn what was thought to be well established. It is
this sociological phenomenon that makes it so difficult to climb back up
the decision tree to the faulty branching.

Once one returns and takes the correct branch, the way forward is not
necessarily obvious. The host of questions which had been assigned
seemingly reasonable explanations along the faulty branch must
be addressed anew. And there will always be those incapable of surrendering
the old world view irrespective of the evidence.

In my opinion, the newsuccesses
of MOND can not occur by accident.
They are a strong sign that we are barking up the wrong tree with
dark matter. A grander theory encompassing both MOND and General Relativity
must exist, even if no one has as yet been clever enough to figure it out
(few have tried).

These all combine to make life as a cosmologist interesting.
Sometimes it is exciting. Often it is frustrating. Most of the
time, ``interesting'' takes on the meaning implied by the old Chinese curse: